1. Field of the Invention
This invention relates generally to the manufacture of semiconductor devices, and in particular to devices with laser-configurable fuses that can be selectively blown to achieve different circuit functions and capabilities.
2. Description of Related Art
Due to the ever-increasing number of applications and uses for integrated circuits, two primary objectives for IC manufacturers are the ability to customize circuits for specific uses and the ability to quickly turn around a circuit prototype to the customer. One method of customizing or configuring circuits is to utilize laser fuses to alter the structure, path, or electrical characteristics of the semiconductor device. Laser fuses, which also refer to antifuses, can also be used to repair memory elements. In particular, fuses have been used to: (1) repair non-functional devices through the selective deletion of defective portions of the circuitry or the substitution of functional redundant circuitry for the defective portions of the circuitry; (2) mark the device for identification of characteristics in a manner that is readable visually or electrically, e.g., serialization of the integrated circuit or how the device has been configured by the laser; and (3) customize an integrated circuit such that the integrated circuit has specific circuit or electrical characteristics.
Typically, the fuse elements are disconnected (blown) by irradiating the fuse with a targeting energy beam, hereinafter generally referred to as a laser, or by applying an electrical current to the fuse. Suitable materials for the fuse include but are not limited to Al, AlCu, AlSiCu, Cu, Ti, TiN, TiW, W, WSi, polycrystalline Si, and TiSi. It is common for the fuse to be covered by an insulating layer of silicon oxide, silicon nitride, or other insulating materials, which are applied as inter-conductive dielectric layers, and/or as part of a final passivation layer to protect the device from moisture and scratches.
The fuse disconnection process begins when the fuse body is heated by the laser, resulting in a change in the fuse material from a solid state to a liquid, vapor, or liquid/vapor state. The insulation layer covering the fuse is helpful to this process because the insulation layer retains heat and results in a more complete and uniform vaporization of the fuse material. Preferentially, the resulting pressure from the vaporization of the fuse causes the overlying insulation to be xe2x80x9cblown openxe2x80x9d or rupture, allowing the vaporized fuse material to escape, and thereby completing the disconnection. If the insulation layer is too thin, the insulation layer may rupture prematurely, i.e., before the heat has propagated through the fuse body, thereby allowing heat to escape and resulting in an incomplete vaporization of the fuse (underblown). On the other hand, if the insulation layer is too thick, the insulation layer may rupture late or not at all, which can result in structural damage to the insulating layer and occasionally to surrounding circuit elements (overblown). The optimal thickness of the insulation layer varies with its composition and the fuse characteristics, but is generally maintained between 1000 xc3x85 and 6000 xc3x85, and should be approximately uniform over the fuse elements across the circuit to increase the repeatability of the fuse disconnection process. Since this is thinner than most dielectric or passivation layers, an opening or window is typically etched into the layer to provide the desired thickness over the fuses. Variations in the thickness of the fuse material, the registration of the laser spot with respect to the center of the fuse, and other factors can also result in the fuse being underblown or overblown.
FIG. 1A shows a top view of a laser-configured fuse structure 10 formed on and covered with a layer of insulating material, such as a silicon oxide 11. Fuse structure 10 includes a fuse body 12 and two fuse terminals 13, which are connected to underlying circuit elements. Fuse body 12 is blown to sever the connection between the underlying elements, creating a disconnection hole 14 in the oxide 11. FIGS. 1B and 1C are cross sectional views of the device in FIG. 1A along sectional lines A-Axe2x80x2. FIG. 1B shows a situation where the fuse blowing process results in an underblown fuse. Because the fuse body is not completely blown (e.g., due to insufficient heat diffusion), a portion 15 of the fuse body may remain in the corner or other areas of the disconnect hole 14. If the portion 15 extends to both fuse terminals, an electrical connection between the fuse terminals still exists. Since the disconnection was not completed, the circuit will not function as intended. FIG. 1C shows a situation where the fuse blowing process results in an overblown fuse. In this case, a crack 16 can form along an edge or edges of the disconnect hole 14. The crack may be formed from the pressure of the vaporizing fuse or from molten material being forced into the oxide 11 (commonly referred to as a xe2x80x9chillockxe2x80x9d). Residual metal 17 from the fuse body may remain in the crack 16 to electrically connect the two fuse terminals, with the result that the circuit will not function as desired. Thus, when a fuse is underblown or overblown, residual fuse material may remain which retains an undesired electrical connection between the fuse terminals.
A technique used to improve the yield of circuits that utilize laser-configured fuses is to follow the laser configuration step with an etching step, which removes at least a portion of the residual fuse material and completes the disconnection. This etch can either be performed in a plasma etcher or through the use of wet chemicals. It is essential that the circuit be protected by a passivation layer or other protective layer that has a relatively slow etch rate compared to that of the fuse material, so that other circuit elements formed from the same material or materials having a similar etch rate to that of the fuse are not etched away or damaged during the post-laser etching.
However, the passivation layer may hinder testing of circuits that require testing prior to laser configuration, such as microprocessors with memory or configurable logic, fuse-programmable ASICs and other logic devices, and other circuits which provide redundant elements to serve as replacements for defective circuit elements. Testing is performed prior to laser configuration to identify which circuit areas that are defective and should be disconnected from the circuit, and which redundant elements need to be connected to replace the defective ones. Testing typically involves applying test probes to the surface of bond pads to provide an electrically conductive path between the tester and the circuit elements. In order for the test probes to make good electrical contact with the bond pads, the bond pads must be exposed and free of oxide or other passivating material. However, if the bond pads are comprised of material that has similar etching characteristics to the fuse material, the passivating material is needed to protect the bond pads from damage due to the post-laser etching. Without the passivating material, the bond pads may be etched and become eroded during the post-laser etching. Depending on the degree of the post-laser etching, the top layer of the bond pad may be either completely or partially etched away. In either case, problems may arise when attempting to connect bond wires to the bond pads. The bond wires may either fail to adhere to the bond pads during the bonding process, or they may break loose from the bond pads at a later date. Thus, the quality of the connection between the bond wires and the bond pads will be degraded, resulting in possible faulty, unreliable, or sub-standard devices.
In order to avoid an undesirable etching of the bond pads, photoresist may be applied to the circuit and patterned in such a way as to cover the bond pads, but leave the fuse regions uncovered. This requires the additional time and expense of making a mask to form the desired pattern on the photoresist and of performing another masking process step, thereby increasing the cost and decreasing the yield of the device. Furthermore, any residual resist left remaining over the unvaporized metal between the connection terminals will protect the metal from the post-laser etch. As a result, the resist will prevent the remaining metal from being removed, which prevents the desired electrical disconnection to be completed.
Accordingly, it is desirable to have the ability to perform testing and post-configuration etching without damaging the bond pads that will be bonded to wires without the time and expense associated with further masking and processing steps.
In accordance with the present invention, a bond pad is provided having a primary bond pad region and a secondary bond pad region, where the secondary bond pad region is contacted by a probe during testing. Both bond pad regions are initially covered with an oxide. Prior to testing, the oxide is retained over the primary bond pad region, but removed over the secondary bond pad region. The secondary pad region is electrically connected to its associated primary bond pad. Thus, the device can be tested for functionality by applying a probe to the exposed secondary pad region. If the device is non-functional but can be repaired by invoking redundant circuits, laser configuration or some other fuse blowing process is then performed, followed by a post-configuration etch to complete the desired disconnections. The post-configuration etch will damage the secondary bond pad region, but will not affect the primary bond pad region because it is protected by the oxide. The oxide can then be removed over the primary pad region and the device given a final test for functionality. If the device is functional, it can be packaged, with the bond wires attached to the undamaged primary bond pad region.
In one embodiment of the present invention, the bond pad area is increased. Thus, the primary bond pad region and the secondary bond pad region form a unitary bond pad structure. A portion of the oxide is removed over the bond pad (over the secondary region) to expose the secondary region for testing. After the post-configuration etch is completed, the remaining oxide is removed from the bond pad (over the primary region), and the wire is bonded to the area previously protected by the oxide.
In another embodiment of the present invention, a separate secondary bond pad is created for every available bond pad on the device. The secondary bond pads are separate from, but electrically connected to, their associated primary bond pads. The oxide is removed over the secondary bond pads to expose them for testing. After the post-configuration etch is completed, the oxide is removed from the primary bond pads, and the wires are bonded to the undamaged primary bond pads.
The present invention will be more fully understood upon consideration of the detailed description below, taken together with the accompanying drawings.